Phenolic waste water from the largest coke coking plant in Poland is treated at a full technical scale. From the very beginning it became evident that very high qualitative variations in short and long periods are to be expected. For this purpose, the biological treatment plant based on activated sludge is protected through preliminary physical-chemical treatment and the results are secured by a final chemical stage of treatment. Nevertheless, improvements in the performance of the treatment plant have been found necessary to introduce. The experienced gained over the last five years has been described and developed improvements have been presented. At the preliminary stage the importance and necessity of preparation was evaluated. It was found essential to use dissolved air flotation, instead of a simple pressured air flotation system. Even with an average relatively high retention in the aeration tanks, there was an uneven distribution of load, dissolved oxygen concentration and oxygen uptake. Dissolved iron or highly dispersed hydroxide particulates in the over flow of the first chemical treatment stage has had a beneficial effect on activated sludge flocs. As a result not only the physical properties but also the overall effect of treatment.